جستجوی مقالات مرتبط با کلیدواژه "sudan low" در نشریات گروه "جغرافیا"

In the Ethiopian-Sudan range forms the low pressure system without front in the cold and transition seasons that is affecting the climate of the adjacent regions by crossing the Red sea. Based on the evidence in the context of Iran, studying Sudan low was first begun by Olfat in 1968. Olfat refers to low pressures which are formed in northeastern Africa and the Red Sea and then pass Saudi Arabia and the Persian Gulf, enter Iran, and finally, cause rainfall. The most comprehensive research specifically examining Sudan low, was the work carried out by the Lashkari in 1996. While he studying the floods that occurred in southwestern of Iran, he was identified Sudan low by the most important cause of such flooding and he explained how they are formed, and how these low-pressure systems were deployed on the southwest of Iran.

The study period with long-term variations was considered from 9.5 to 11 years based on solar cycles. Precipitation data for 13 synoptic stations are considered above 5 mm in south and southwestern Iran. With three criteria were determined for the days of rainfall caused by each type of atmospheric system. The visual analysis of high and low altitude cores and geopotential height at 1000 hPa pressure level (El-Fandy, 1950a; Lashkari, 1996; 2002) were considered based on the aim of the study. Accordingly, the approximate locations of activity centers, as well as the range of the formation and displacement of the Sudan system were initially identified based on the location of the formation of low and high-pressure cores. Then, the rainy days due to the Sudan system in January were separated from the precipitation of the other atmospheric system.

According to the selected criteria in the forty-year statistical period, 507 precipitation systems were identified with different continuities that led to precipitation in the northern coast of the Persian Gulf. The pattern of independent Sudan low rainfall was responsible for 77% of the precipitation in the Persian Gulf. Decade frequency share of Sudan low was lower in the first decade (16%) compared to the next three decades. This system of rainfall was more activated during the second and third decades compared to the first decade. However, rainfall changes were not evident in the mid-decade. Independent Sudan low precipitation provide 25% and 27% of the cold season precipitation of the Persian Gulf during the second and third decades respectively. In accordance with the 24th solar cycle, at the end of the study period, the Sudan low was more effective on the Gulf coast than ever before. During this decade, 125 cases of Sudan low rainfall was recorded for the Persian Gulf. Thus, the frequency of Sudan low during the fourth decade was about 31%, which was higher than in the rest of the decade. Overall, the Sudan low rainfall was repeated 151 times for 2 days rainfall, during the statistical period studied. This Precipitation has increased over the last decades compared to other periods.

The severe variability of rainfall along the timing and location of the permanent Persian Gulf coasts can have a significant impact on the economic and agricultural behavior of the Gulf population in the three provinces of Ahwaz, Bushehr and Hormozgan.The purpose of this study was to evaluate the precipitation changes due to Sudan low in the Persian Gulf coastal region during the cold period. The results of this study showed that the role of integration patterns in influencing the precipitation of the Persian Gulf coast has decreased with the strengthening and further activation of the Sudan low system during the last two decades. That way, about 77percent of the region's rainfall is provided by independent Sudan low. At the end of the course (in accordance with 24th solar cycle activity) the Sudan low system was more active than before. Although the Sudan low activity was different at each station during the period studied, but in the historical passage incremental and decade's positive behavior of Sudan low was common to all stations. Evaluation of changes in rainfall duration shows that the pattern of precipitation with 2days duration is more frequent than the patterns of one to several days.

The Sudan low pressure is regarded as a system influencing the rainfall during the cold season, in the South and Southwest of Iran. Often in the cold season these systems have crossed Ethiopia, Sudan, and the Red Sea and then on its way entered the country from the south and southwest of Iran and it is causing rain in these areas. According to El-Fandy (1948), the history of recognizing the Sudan low in the Middle East and the Red Sea region goes back to about 80 years ago when Ashbel (1938) first described the eastern Mediterranean rainfall. Ashbel concluded that the rainfall in the area was affected by a system which he called the “Red Sea low pressure”. Based on the evidence, Olfat (1968) was the first one who studied Sudan low in the context of Iran. Olfat refers to low pressures which are formed in northeastern Africa and the Red Sea and then pass Saudi Arabia and the Persian Gulf, and finally, enter Iran and cause rainfall. The purpose of this study is the Investigation of dynamic fluctuations of Sudan low In May rainfall performance over the course of 60 years.

The study period is from1957 to 2017. May was considered as a symbol of the poor performance of the Sudan low in the south and southwest of Iran (Lashkari & Mohammadi, 2019). The study period with long-term variations was considered from 9.5 to 11 years based on solar cycle. In this regard, the daily precipitation data of 42 stations of the south and southwest of the country were prepared by the Meteorological Organization of Iran. Rainfall days were extracted in May using the daily rainfall data of 42 synoptic stations. For these days, sea level pressure (SLP) and geopotential (hgt) data at 1000 hPa with 2.5 × 2.5◦ spatial resolution were obtained from the dataset of NCEP/NCAR reanalysis project. Additionally, the frame of the reference was provided in 0-100◦ E and 10-55◦ N latitude belt in GrADS software. The visual analysis of high and low altitude cores and geopotential height at 1000 hPa pressure level (El-Fandy, 1950a; Lashkari, 1996; 2002) were considered based on the aim of the study. Accordingly, the approximate locations of activity centers, as well as the range of the formation and displacement of the Sudan system were initially identified based on the location of the formation of low and high-pressure cores. Then, the rainy days due to the Sudan system in May were separated from the precipitation of the other atmospheric systems (i.e., Sudan-Mediterranean and Mediterranean systems). For these days, SLP, hgt, specific humidity, zonal and meridional wind data at the levels of 1000, 850, 700, and 500 hPa with 2.5 × 2.5◦ spatial resolution were obtained from the NCEP/NCAR dataset. Using the GrADS software, the numerical values of these parameters were calculated at synoptic stations with the highest statistical data over six decades. In addition, for the sum of the days of Sudan low rainfall in each decade, the daily composites of the anomalies of these variables were provided from NCEP/NCAR dataset.

Analysis of sea level pressure changes Examination of sea level pressure (SLP) revealed that during the first to sixth decades SLP has declined relative to its long-term average in southwestern of Iran (-1 to -3 hPa). But in the south of Iran during the first to second decades, the SLP decreased slightly and remained constant in the third decade and then increased during the fourth to sixth decades (+1 hPa).Analysis of Geopotential height Changes Examination of Geopotential height (hgt) changes over the level of 1000hpa to the 500hpa showed that the height of the Sudan low was reduced Compared to its long-term average, during the first decade to the sixth decade in southwest of Iran but the size of its hgt reduction, was reduced from the first decade (-70 m) to in the sixth decade (–30m). The height of the Sudan low was decreased from the first decade to the third decade in the southern part of Iran (-10m to -30m). But the height of the Sudan low was increased during the fourth to sixth decades (+10 to +15 meters).Analysis of specific humidity Changes Examine the specific humidity (Shum) values at 1000,850 and 700hpa showed that moisture change was reduced during the first to the second decade in southwestern Iran (about 2 gr/kg), and it was constant with a slight swing from the second decade to the sixth decade (4 gr/kg). In the south of Iran Shum status was accompanied by decreasing and increasing with a swing during the first decade to the third decade and it was almost fixed from the fourth through the sixth decades (2.5-3 gr/kg). Specific humidity values increased to some extent at 850 and 700hPa during the last decade.Analysis of vector wind and vorticity ChangesSurvey of vector wind and vorticity at 850hpa level showed that Sudan Cyclone deployed in northeast Saudi Arabia and the southern coast of the Persian Gulf and on other hands, Anticyclone (Arabian subtropical High Pressure) Stationed on southern Iran. Soduring the decades of climate, the Barotropic and Baroclinic Atmosphere is ruler on the south and southwest of Iran, respectively.

The purpose of this study was to investigate the performance of a compressive structure, humidity and other dynamic parameters of systems due to Sudan low in May of south and southwest of Iran. The climate decades were regulated by solar cycles over 60 years (1957-2017). The results made it clear that changing the dynamic parameters of Sudan low system has strengthened and weakened in the southwest and south of Iran respectively, over the decades. Also, Investigation of the average of decidedly precipitation rate due to Sudan low showed the values of rainfall in May, increasing in the south and southwest of Iran over the past decades until now in a long historical trend.

  Although precipitation is of great importance in all climates, it plays a vital role in the arid and semi-arid regions. The spatial and temporal distribution of precipitation in all climates is affected by special synoptic structures in which one or two systems play the controlling role. The southern Iran is located adjacent to two important climate systems whose spatial arrangements determine the timing and amount of precipitation in the mentioned region. Therefore, it is important to study the possibility of predicting the drought and wet years in this geographical region of Iran according to its strategic role in the ecology, agriculture, industry, transportation, and politics. The study was conducted on Hormozgan, Boushehr, Kohkilouyeh-va-Boyerahmad, Chahar-mahal-va-Bakhtiari, and Khuzestan Provinces in Iran. The daily precipitations in the selected stations were extracted, harmonized and arranged in a 30 year statistical period. Then, the situation of each station was determined from the viewpoint of drought and humidity using the SIPA criterion and DIP software. We have selected the years in which intense wet years were half of the selected stations, based on the mentioned criteria. These years have been selected as the samples of intense wet years. The atmospheric data of the mentioned years were extracted from the website http://www.esrl.noaa.gov, and the daily maps of these years were created at levels of 1000 and 500 hPa in the longitude of-40 degree west to 100 degree east and the latitude of zero (the equator) to 80 degree north using Grads software. The Arabian subtropical high pressure nuclei were determined for all days and their maps were created as the output maps using ArcGIS10.3 software. The data were reproduced in a matrix with the dimensions of 67×2145 based on the daily precipitation of more than 5 millimeters. The study area, located between the latitudes 0 to 80 degree north and -40 to 100 degrees east, has a number of days according to the spatial data resolution which was 2.5×2.5 geographical degree. Afterwards, justification of the data distribution according to the special values, variance percentage and accumulation variance was determined for analysis of the factors. Just 12 factors had the values larger than 1 in the primary analysis. The principle component analysis and Varimax rotation showed that concentrating on the correlation of 13 factors can explain 89.18 percent of the pattern’s behavior.
Finally, the dominant patterns in the selected intense wet year samples were extracted through studying the maps of 1000 and 500 hPa from the twelve extracted factors. Then, we have analyzed the maps of subtropical jet stream, divergent and convergent flux, special moisture, temperature blow, and etc. Moreover, the maps of different levels in all rainy days of the intense wet years were reviewed. Comparison of the repetitive patterns resulting from the review and the principle factor analysis provided similar results. Our study showed that in all the rainy days, the central nucleus of the Arabian anticyclone cell at all levels of 850 and 700 hPa was located in the east part of the longitude 45 degree E. When the anticyclone central nucleus is located in the east part of the longitude 55 degree E, the precipitations are more extensive, and cover all the regions from Khuzestan to Hormozgan. As it can be seen, in the rainy days, one or two divergent flux nuclei are located on Oman Sea or western Arab Sea and Gulf of Aden. In such condition, the streams in the lower levels of troposphere (from sea level to 850 hPa) are changed into eastern streams in northern Oman Sea and gradually on the Arab Sea. This condition is the most suitable mechanism for moisture advection towards the Sudan low pressure. In the same day, two strong nuclei of convergent moisture flux are dominated on Ethiopia and central Arabia which receive the moisture transmitted from the warm seas. The highest moisture advection towards the Sudan system is done under the layer of 850 hPa from Arab Sea, Oman Sea and the Gulf of Aden. Because of the topographic condition at levels higher than 850 hPa, this advection comes much lower, and the moisture transmitted from the transition branch may add to this moisture from the tropical convergence region. With the moisture advection in the lower levels, proper thermodynamic condition is provided for the development of convection clouds on the region. These clouds initially appear as mass clouds on Sudan and Red Sea, and then on Arabia, and then gradually move to Iran through the southern streams before trough at levels of 850 hPa and higher. Then, the clouds grow and extend through involving in the upward streams dominated on the front trough and under the subtropical jet stream, which are located on Red Sea and northwest Arabia. The proper temperature blow and diabatic warming resulting from condensation process provide a severe condition in the northern part of the jet. In order to have an intense wet year in the south and south west of Iran, the eastern movement of Arabian high pressure is considered as an important factor. With the eastern movement of this high pressure, a proper synoptic condition for advection of moisture toward the precipitation system is provided. A proper condition is also provided for the extension of the Mediterranean trough towards the southern latitudes on the south east part of African desert and cold advection on the region at the middle and upper layers of troposphere. The Arabian high pressure has very high ability for wet condition, especially at the lower layers, because of its dynamic structure. Therefore, the moisture moved through the divergent flux toward the southern systems is considerable and provides significant potential energy for the convection systems. The cold advection from the northern latitudes and the warm advection from the subtropical latitudes provide proper heat gradient for intensification of the subtropical streams in the northwest domain of the Arabian high pressure. These jet streams are formed in the limits of northeast Arabia and provide proper dynamic condition for intensification of intense convection streams in the Northern Arabia and Southern Iran. The convection clouds cause intense precipitations on the region because of the access it has to the moisture of the southern warm seas and also the moisture moved from the northern branch of the tropical convergence region

In this research large scale dynamic and thermodynamic anomalies during storm events resulted from cyclogenesis in the Mediterranean Sea and Sudan low-pressure systems، are investigated. 5 severe storms that happened in KarunBasin (1998-2008) were selected and using Japanese Re-analysis data (JRA25)، anomalies of dynamic and thermodynamic indices، 2 days prior to the start of the storm until the end of the storm were analyzed. The selected indices in this research are potential vorticity، convergence and divergence، vertical velocity، absolute vorticity advection، specific humidity، moisture flux، potential temperature and equivalent potential temperature. According to the results and comparing 6-hourly recorded rainfall amounts، it was found out that in the reference events، before the start of the storm، geopotential height values، in the under-study region decreased and at the time of maximum rainfalls، the geopotential height reached to its lowest values and by end of the storm، the values started increasing، whereas parameters related to convergence and vertical movements، such as potential vorticity، vorticity advection، moisture flux، convergence of moisture and specific humidity amounts corresponded to the same trend of rainfall from the beginning to the end. It is obvious that none of these indices can individually cause the occurrence of a storm، but by analyzing trends and regressions، it seems that there are meaningful relationships between geopotential height، moisture advection and potential vorticity and rainfall amounts which can be used in forecasting future rainfall events. To verify the results obtained from the reference events، 2 days without rainfall at least 2 days before and after the selected days، were also selected and studied. The results verify considerable changes of the selected dynamic and thermodynamic indices during stormy days compared to the days without any rainfall in the region.

This study aimed to identify the cause of intense rainfall occurred on April 30, 1994 in Kermanshah, The maps of surface pressure in the upper atmosphere Sudan and low pressure systems result in precipitation patterns were identified The role of this system as one of the major systems of the West was to climax. Maps and atmospheric pressure at the upper and lower levels, it was determined that Starting with the establishment of precipitation over the Mediterranean trough Turkey and Iraq and the location of the study area at the front of the upper levels, Sudan is associated with low pressure at the surface occurs. There is a high pressure system over the Arabian Peninsula and North-West Indian Ocean This system leads to amplification. Omega plans and volubility of the story of instabilities weather is rainy day. The moisture content of the maps, Most moisture levels 850 hPa and the Mediterranean Sea to be supplied the region is imported from the West. But the 700 hPa level is the main source of water that flows from the Red Sea to the south west of the study area appears.

In order to study of dust storm of Bushehr Province¡ all dust storms events collected and recognized as different patterns. The 24 February 2010 event selected as study sample and using graphical and calculational method¡ speed and direction in sea level and 500 hPa¡ vertical lifting speed of air mass from ground to 500 HPa maps prepared and analyzed. The result of this study indicated that in the study period¡ occurrence of blocking and vorticities in 500¡ 600 and 700 hPa¡ intensity instability and convection in all atmospheric levels from 1000 to 500 hPa and positive vorticities in 500¡ 600¡ 700 and 850 hPa observed. Also creating an anticyclone system in north of Caspian Sea obstacles of movement of this blocked system to east. At this time in Sudan and south of Saudi Arabia prevalent an intensity thermal convection system.

In order to investigate some synoptic aspects of Sudan low systems such asupper and lower level tropospheric circulation pattern and moisture source and theiradvection way, the synoptic pattern on the occurrence time of December 2001 stormover Iran has been noticed. Therefore the maps of surface, 500, 200, 50hpa levelsbefore and after storm were studied. In order to understand the situation of subtropicaljet stream and how it strengthens, maps of zonal wind component and velocitypotential (x) of 200hpa level have investigated. Also, satellite images of cloud form ofstorm have been studied.The results showed that although like previous studies, there was low cycle indexand existence of a ridge in mid-troposphere over the west Mediterranean and a deeptrough over the east Mediterranean had important role on the formation and evolutionof Sudan low; but the main role belonged to the stretching of polar vortex in lowerstratosphere as the main source of vorticity for abnormal circulation and strengtheningof Hadley cell and Subtropical jet stream over Mediterranean and north of Africa.The results indicated that the suitable position of Subtropical jet stream core overMiddle East associated with the settlement and strengthening of a ridge in middle andlower troposphere over the Arabian Sea are necessary for the evolution and developingof Sudan lows.However, the position and speed of subtropical jet stream core over the MiddleEast and it’s axis direction in the upper troposphere control both the mid-troposphericcirculation pattern and the tracks of incoming Sudan lows to Iran.The consequences indicated that settlements and strengthening of ridge/high over theArabian Sea, while making a set positive feedback and subsequently intensify Hadleycell and strengthening subtropical jet stream over Red Sea region, lend to directtransportation of moisture from the Arabian Sea into Sudan lows.Moreover, it was found that the main sources of moisture of Sudan systems aretropical Eastern Africa and southwest sector of Arabian Sea which strengthening whenthey pass over the Read Sea and Persian Gulf.